Method of manufacturing sandwich panel

ABSTRACT

The invention relates to a method of manufacturing a sandwich panel comprises the steps of: 
     a) providing a plate-shaped assembly of a first cover part and a second cover part and between these cover parts a core part of a thermoplastic material containing a physical blowing agent, 
     b) heating the assembly resulting from step a) under pressure between press tools in a press to a foaming temperature below the glass transition temperature of the thermoplastic material in the core part, thereby effecting adhesion of the foamed core part to the first and second cover parts 
     c) foaming the thermoplastic material in the core part under pressure and at the foaming temperature wherein the spacing between the press tools is increased; 
     d) a cooling step of cooling the foamed sandwich panel resulting from step c), while the sandwich panel is maintained under pressure between the press tools; 
     e) removing the thus cooled sandwich panel from the press; and 
     f) drying the sandwich panel thus obtained; 
     wherein the cooling step d) comprises a first substep d1) of cooling the foamed assembly from the foaming temperature to an intermediate temperature in the range of 70-100° C. at a first cooling rate and a second substep d2) of cooling the foamed assembly from the intermediate temperature to ambient temperature at a second cooling rate, the second cooling rate is less than the first cooling rate.

-   -   This application is the National Stage of International        Application No. PCT/NL2016/050488, filed 7 Jul. 2016, having the        title “METHOD OF MANUFACTURING A SANDWICH PANEL” which claims        the benefit of and priority to Netherlands Application No.        2015138, filed on 10 Jul. 2015, the contents of all of which are        incorporated by reference as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a sandwichpanel, comprising a foamed core part between two cover parts, accordingto the so-called in situ foaming technique.

BACKGROUND

EP 636463 A1 has disclosed this so called in-situ foaming technique.This known technique comprises the steps of providing a sheet of athermoplastic material comprising an amount of a suitable physicalblowing agent (a swelling agent or solvent), placing this sheet betweentwo fibre-reinforced cover layers of a similar thermoplastic material,placing the assembly of thermoplastic core and fibre-reinforced coverlayers between two heated press plates, supplying heat and pressure tothe assembly and upon reaching a foaming temperature, causing foaming ofthe thermoplastic core by increasing the spacing between the pressplates, cooling the press plates when a predetermined foamed corethickness is obtained, while the sandwich panel thus obtained is keptunder pressure, followed by a drying step to reduce the content ofremaining physical blowing agent or solvent.

WO 2006080833 A1 has disclosed that during the drying step at elevatedtemperature of the in situ foaming technique the remaining physicalblowing agent is preferably removed, while the outflow thereof via theperipheral edges of the foamed core is restricted.

It is also known from a thesis “The development of in-situ foamedsandwich panels” of P. W. C. Kluit, Delft University Press, 1997, page63, that optimized parameters regarding mechanical properties for thein-situ foaming of PEI using acetone as a physical blowing agent were:

Acetone weight percentage: 11.5-12.5

Foaming temperature: 180° C.

Initial pressure: 3 MPa

Heating time: 20 sec

Opening speed press: 0.4-0.5 mm/sec

Final height: 10-11 mm

Cooling rate: 100° C./min until 90° C., followed by 20° C./min until 20°C.

Kluit explains that experiments for 25×25 cm sandwiches had shown that arapid cooling at 100° C./min from the foaming temperature to ambienttemperature always resulted in a bad adhesion. Based on experimentalresults an intermediate temperature of 90° C. for decreasing the coolingrate was chosen in view of adhesion strength between the foamed corepart and the cover part. Although these results were promising, thisadhesion is still poor locally. This became more evident upon upscalingto larger dimensions of the sandwiches. Upon upscaling the strengthvalues cited by Kluit were not achieved.

From WO 86/04017 A1 a method of manufacturing blocks of polyurethane orother open-cell foam by an exothermic reaction of starting materials,wherein once the reaction has reached a desired stage of completion acooling gas is passed through the body of the block to carry away theheat of reaction until a stable temperature is reached.

SUMMARY

Therefore the invention aims at providing a sandwich panel of thedescribed nature in accordance with the in situ foaming technique,wherein the adhesion between the foamed core part and the cover parts isimproved.

Accordingly, the method according to the invention of manufacturing asandwich panel comprises the steps of:

a) an assembling step of providing a plate-shaped assembly of a firstcover part and a second cover part and between these cover parts a corepart of a thermoplastic material containing a physical blowing agent;

b) a heating step of heating the assembly resulting from step a) underpressure between heated press tools in a press to a foaming temperaturebelow the glass transition temperature of the thermoplastic material inthe core part, thereby effecting adhesion of the foamed core part to thefirst and second cover parts;

c) a foaming step of foaming the thermoplastic material in the core partunder pressure and at the foaming temperature wherein the spacingbetween the press tools is increased;

d) a cooling step of cooling the foamed sandwich panel resulting fromstep c), while the sandwich panel is maintained under pressure betweenthe press tools;

e) a discharging step of removing the thus cooled sandwich panel fromthe press; and

f) a drying step of drying the sandwich panel thus obtained;

wherein the cooling step d) comprises a first substep d1) of cooling thefoamed assembly from the foaming temperature to an intermediatetemperature in the range of 70-100° C. at a first cooling rate and asecond substep d2) of cooling the foamed assembly from the intermediatetemperature to ambient temperature at a second cooling rate, wherein thefirst cooling rate is at least 140° C./min and wherein the secondcooling rate is less than the first cooling rate.

In the method according to the invention first a plate shaped assemblyis prepared by stacking a first cover part, a core part made of athermoplastic material containing a sufficient amount of blowing agentfor foaming to the final thickness achieved in subsequent steps, and asecond cover part, onto one another. Typically these parts will bepresent as sheets or films. The plate-shaped assembly is usuallyflexible and adapts to the shape of the press tools, which may be flatin order to produce flat (planar) sandwich panels. A more complex shapeof the press tools such as curved or double curved in differentdirections, e.g. for manufacturing a roof of a car or an interiorsidewall panel for an aircraft, is also contemplated. Typically thepress tools such as flat press plates are releasably mounted in thepress.

The assembly is heated to a foaming temperature between heated pressplates in a pressurized condition in order to prevent prematureexpansion of the core part and in order to simultaneously generatebonding of the core part to be foamed to the cover parts. At the foamingtemperature the press is opened in a controlled manner, thereby allowingthe core part to foam. Upon reaching the predetermined thickness of thepanel determined by the height of the foamed core part the thus obtainedsandwich panel is cooled according to a multistep process. However,compared to the known prior art two step process, the cooling rate ofthe first substep between the foaming temperature and the intermediatetemperature is much higher. The second substep can be performed underconditions known from the prior art.

It has appeared that by increasing the cooling rate in the first substepthe adhesion is improved significantly and local defects are less likelyto be present.

The method according to the invention can be performed using anythermoplastic plastic material in the core part, which thermoplastic canbe foamed by a blowing agent. Examples of suitable thermoplasticsinclude polyetherimide (PEI), polyethersulfone (PES), polysulfone (PSU),polyphenylenesulphide (PPS), polyphenylsulfone (PPSU), polyketone,liquid crystal polymers, polycarbonate (PC), propylene etc. A preferredthermoplastic for use with a physical blowing agent is polyetherimide(PEI).

The core part contains an amount of physical blowing agent, that issufficient for foaming the thermoplastic material in the core part tothe desired final thickness. This thickness is determined by the finaldistance achieved between the press tools in the foaming step c) andcooling step d). Typical examples of the physical blowing agent includelow boiling organic compounds. A preferred example is acetone.

In the foaming step a closed cell foam is formed, typically ananisotropic foam with elongate cells that are oriented in the heightdirection (that is to say the largest dimension of the cells extend in adirection from one cover part to the other cover part).

The cover parts can be suitably selected from sheets of thermoplasticmaterial, metals and combinations thereof. Suitably the thermoplasticmaterial, if any, of a cover part is the same as the thermoplasticmaterial of the thermoplastic core part. Suitable thermoplasticmaterials include polyethersulfone (PES), polyphenylsulfone (PPSU) andpolysulfone (PSU), in particular polyetherimide (PEI) in view of theirfavourable flameretarding properties. However, combinations of differentthermoplastics are also contemplated. Suitable examples thereof compriseinter alia PEI core part between cover parts, wherein at least one ofthe cover parts is made from PS or PC, and a PES core part and at leastone PC cover part. Aluminium is a preferred metal for a cover part inview of weight. In view of weight and strength in an advantageousembodiment at least one of the first and second cover part comprises oneor more consolidated layers of a fibre-reinforced thermoplastic.

Here it is noted, that if a cover part comprises multiple sublayers of(fibre-reinforced) plastic material, these layers are consolidated (thatis to say subjected to a heat treatment above the glass transitiontemperature in a pressurized condition), such that the(fibre-reinforced) thermoplastic sublayers are irreversibly adhered toone another and form a single integral cover part. This consolidationstep is necessary as during the foaming step no bonding would occurbetween these layers, as the physical blowing agent cannot diffuse fromthe core part through an adjacent layer onto the interface between thelayers of the cover part.

Glass fibres are a preferred example of reinforcement, if present in acover part. However other inorganic fibres, such as metal fibres, carbonfibres and organic fibres like aramid fibres, can be applied. Inaddition to the above synthetic fibres natural fibres can also be used.The fibres in the reinforcement of a cover part may optionally beoriented, and there are no restrictions whatsoever on the length andorientation. Knitted fabrics, woven fabrics, mats, cloths andunidirectional fibres represent various manifestations thereof.

The heating step, foaming step and drying step are performed underconditions similar to those disclosed in the above mentioned state ofthe art documents, depending on the starting materials and dimensions.Typically the pressure during the heating step, foaming step and coolingstep is in the range of 3-5 MPa. Higher pressures are also contemplated.In heating step b) the assembly is arranged in the press, which ispreferably preheated. Upon heating the assembly between the press toolsthe temperature of the assembly reaches the foaming temperature (e.g.175-182° C. for a PEI core part), whereafter the distance between thepress plates is increased to a predetermined value. In cooling step d)the foamed assembly, while kept in the press under pressure (usuallyessentially the same pressure as during foaming) is cooled down toambient temperature as explained above. In a first cooling step d1) thesandwich is cooled at a cooling rate of at least 140° C./min, preferablyover 200° C./min, more preferably more than 240° C./min until theintermediate temperature, e.g. 90° C. Subsequently a second coolingtreatment is carried out wherein the foamed sandwich panel is furthercooled from the intermediate temperature at a much lower average coolingrate, preferably less than half of the first cooling rate, morepreferably at 25° C./min or less, such as 20° C./min. If the coolingrate in the first cooling step is less than 140° C., then adhesion ispoor. High cooling rates above 200° C./min, such as over 240° C./minprovide better results.

After unloading the thus obtained sandwich panel from the press, thesandwich panel is subjected to a drying treatment. This drying treatmentis preferably carried out by increasing the temperature in intervals upto a temperature in the range of about 150° C. to about the glasstransition temperature of the foamed core thermoplastic. For PEI the Tgis 217° C. The temperature increase between intervals is usually about10 degrees. The sandwich panel is maintained at each intermediatetemperature for a sufficient period of time, for example two hours.Advantageously the drying step e) is initiated within 10-12 hours afterthe end of the foaming step b). If at least one of the cover partscomprises a thermoplastic material the drying is preferably carried outas disclosed in WO 2006/080833 A1.

The sandwich panels obtained using the method according to the inventioncan be further processed, for example shaping to the desired final shapeby edge finishing. The sandwich panels made in accordance with thepresent invention are advantageously used in light weight applicationswhere fireproof properties and/or sufficient strength/stiffness arerequired. A preferred application area is the transport sector, inparticular the air- and spacecraft industry.

In order to achieve the high cooling rate in the first substep d1)typically the press plates made of tool steel is provided with parallelbores, wherein a forced flow of cooling water is generated in oppositedirections in adjacent bores. Now it has appeared that if there aretemperature differences in the plane of a press tool (non-homogeneouscooling) the appearance of the obtained sandwich may be poor, inparticular the positions of the cooling bores might be reflected andmight be visible as discoloured areas. Moreover, the adhesion betweencover part and foamed core part might not be homogeneous inducing therisk of local failure at that interface instead of within the foamedcore part.

Therefore during substeps d1) and d2) the temperature difference betweenthe press plates is preferably less than 2° C. (measured 4 mm below thesurface of the press plates).

In some cases depending on factors like dimensions of the sandwichpanel, the construction material of the press, cooling capacity and thelike it has appeared difficult to reduce the temperature of a press toolhomogenously (meaning that there is substantially no temperaturedifference over the surface of a press tool). In such conditions it hasproven to be useful to provide a layer of material having a heatconductivity coefficient higher than that of the construction materialof the press. A layer of copper or aluminium, such as a woven matthereof, is a preferred example. Such a mat or sheet can be easily bearranged at both sides of the assembly in step a) of the processaccording to the invention, or be inserted in the press itself. Thislayer of material having a high heat conductivity coefficientcontributes to equalizing the temperature of the press during cooling,thereby reducing the temperature variations over the surface of a presstool and reducing the temperature difference between the press tools.This embodiment contributes to a homogeneous cooling with the resultthat no traces from the cooling bores of the press plates are visible inthe sandwich obtained.

EXAMPLES The invention is further illustrated by means of the followingExamples. Example 1

First and second cover parts: each one layer US-style 7781 glass fabricPEI (polyetherimide) impregnated and consolidated with 33+−2% PEI, layerthickness=0.23 mm;

Thermoplastic core part: two films of PEI, (Polyetherimide) Ultem 1000,impregnated with 12,1-12,9 wt. % acetone, film thickness in the range of250-300 micrometres.

The percentage of acetone in the film is determined as ((weight offilm+acetone in g) minus (weight of the neat film in g)) divided by(weight of the neat film in g).

Several FITS panels (planar dimensions 50×30 cm) were manufactured withthe following configuration:

A symmetrical stack was assembled with the two acetone impregnated PEIfilms as core part between the identical first and second cover parts,either each consisting of one or two glass fabric layer(s) as indicatedabove. This assembly was placed between the heated press plates of thepress. After closing the press the assembly was heated in seconds to therequired foaming temperature of 178-180° C. The centre of thetemperature measuring device (Pt element type K) is located 4 mm belowthe surface of the press plates. Pressure is 4 Mpa. Upon reaching thisfoaming temperature the press—while maintaining pressure at essentiallythe same value—was opened according to a certain foaming curve to apredetermined thickness (as specified below) of the final sandwichpanel, after which the press plates and consequently the thermoplasticsandwich panel were cooled from the foaming temperature to 90° C. in 25seconds, and further down to ambient temperature at an average coolingrate of 20° C./min. Finally the sandwich panels thus obtained weresubjected to a drying step according to WO2006080833 A1 by taping theedges to reduce peripheral outflow of acetone and direct it through thecover parts using temperature increases of 10° C. between intervals of2-4 hours at a given temperature.

In this way sandwich panels with thicknesses of 9.5 and 7.5 mm weremanufactured. The sandwich panels were tested for the adhesion betweenthe fibre-reinforced thermoplastic PEI cover parts and the in-situfoamed PEI core part using an in plane tensile strength test procedureaccording to ASTM C297.

The 9.5 mm thick in-situ foamed thermoplastic sandwich panel having afoam density of 85 kg/m³ (core part made from 2 PEI films acetoneimpregnated of 300 micrometers) showed an average flatwise tensilestrength of 3.4 MPa. The 7.5 mm thick in-situ foamed thermoplasticsandwich panel having a foam density of 90 kg/m³ (core part made from 2PEI films acetone impregnated of 250 micrometers) showed an averageflatwise tensile strength of 3.9 MPa.

Typically, failure of the test samples occurred in the thermoplasticcore part, indicating that the adhesion between the core part and coverparts is adequate. The cover parts could not be peeled manually from thefoam core.

Example 2

First and second cover parts: each one layer US-style 7781 glass fabricPEI (polyetherimide) impregnated and consolidated with 33+−2% PEI, coverpart thickness=0.23 mm; or each two layer US-style 7781 glass fabric PEI(polyetherimide) impregnated and consolidated with 33+−2% PEI, coverpart thickness=0.46 mm

Thermoplastic core part: three films of PEI, (Polyetherimide) Ultem1000, impregnated with 12,1-12,9 wt. % acetone, film thickness of 250micrometres.

An assembly was prepared from the thermoplastic core part in between thefirst and second cover parts. This assembly was subjected to in situfoaming as outlined in EXAMPLE 1 using the same conditions.

Sandwich panels (25×25 cm) having a thickness of 11.3 mm were obtained.The thermoplastic sandwich panel having cover parts comprising one layerglass fabric impregnated with PEI had a foam density of 87 kg/m³ andshowed an average flatwise tensile strength of 3,5 MPa. Thethermoplastic sandwich panel having cover parts comprising twoconsolidated layers glass fabric impregnated with PEI had a foam densityof 91 kg/m³ and showed an average flatwise tensile strength of 3.9 MPa.

Typically, failure of the test samples occurred in the thermoplasticcore part, indicating that the adhesion between the core part and coverparts is adequate. The cover parts could not be peeled manually from thefoam core.

The same test results were obtained with sandwich panels of 50×30 cm.

Example 3 (Comparative)

First and second cover parts: each one layer US-style 7781 glass fabricPEI (polyetherimide) impregnated and consolidated with 33+−2% PEI, coverpart thickness=0.23 mm; or each two layer US-style 7781 glass fabric PEI(polyetherimide) impregnated and consolidated with 33+−2% PEI, coverpart thickness=0.46 mm

Thermoplastic core part: three films of PEI, (Polyetherimide) Ultem1000, impregnated with 12,1 -12,9 wt. % acetone, film thickness of 250micrometres.

An assembly was prepared from the thermoplastic core part in between thefirst and second cover parts. This assembly was subjected to in situfoaming as outlined in EXAMPLE 1 except that the obtained sandwicheswere cooled from the foaming temperature to 90° C. in 40 seconds.Sandwich panels (25×25 cm) having a thickness of 11.3 mm were obtained.The thermoplastic sandwich panel having cover parts comprising one layerglass fabric impregnated with PEI had a foam density of 87 kg/m³ andshowed an average flatwise tensile strength of 1.8 MPa. Thethermoplastic sandwich panel having cover parts comprising twoconsolidated layers glass fabric impregnated with PEI had a foam densityof 91 kg/m³ and showed an average flatwise tensile strength of 2.3 MPa.

Failure of the test samples occurred at the interface between thefibre-reinforced thermoplastic cover part and the in situ foamed corepart, indicating that the adhesion at the interfaces was less than thestrength of the foam. Also the cover parts could be peeled manually ofthe foam core part rather easily

The same results were obtained with panels having dimensions of 50×30cm.

1-11. (canceled)
 12. Method of manufacturing a sandwich panel comprisingthe steps of: a) an assembling step of providing a plate-shaped assemblyof a first cover part and a second cover part and between these coverparts a core part of a thermoplastic material containing a swellingagent, wherein the thermoplastic material of the core part is anamorphous polymer selected from the group consisting of polyetherimide(PEI), polyethersulfone (PES), polysulfone (PSU), polyphenylsulfone(PPSU), and polycarbonate (PC) and the swelling agent is a swellingagent for the thermoplastic material; b) a heating step of heating theassembly resulting from step a) under pressure between heated presstools in a press to a foaming temperature below the glass transitiontemperature of the thermoplastic material in the core part; c) a foamingstep of foaming the thermoplastic material in the core part underpressure and at the foaming temperature wherein the spacing between thepress tools is increased; d) a cooling step of cooling the foamedsandwich panel resulting from step c), while the sandwich panel ismaintained under pressure between the press tools; e) a discharging stepof removing the thus cooled sandwich panel from the press; and f) adrying step of drying the sandwich panel thus obtained; wherein thecooling step d) is carried out in two substeps, comprising a firstsubstep d1) of cooling the foamed assembly from the foaming temperatureto an intermediate temperature in the range of 70-100° C. at a firstcooling rate and a second substep d2) of cooling the foamed assemblyfrom the intermediate temperature to ambient temperature at a secondcooling rate, wherein the first cooling rate is at least 140° C./min andwherein the second cooling rate is less than the first cooling rate,thereby effecting bonding of the foam core part to the first and secondcover parts.
 13. Method according to claim 12, wherein the swellingagent is acetone.
 14. Method according to claim 13, wherein the firstcooling rate is more than 200° C./min.
 15. Method according to claim 12,wherein the second cooling rate is less than half of the first coolingrate.
 16. Method according to claim 13, wherein the second cooling rateis 25° C./min or less.
 17. Method according to claim 12, wherein duringsubsteps d1) and d2) the temperature difference between the press toolsis less than 2° C.
 18. Method according to claim 12, wherein the surfaceof the press tools is provided with a layer of a material having a heatconductivity coefficient, which is higher than that of the constructionmaterial from which the press tools are made.
 19. Method according toclaim 17, wherein the layer of material having the higher heatconductivity coefficient is a layer made of copper or aluminium. 20.Method according to claim 12, wherein at least one of the first andsecond cover parts comprises a layer of a thermoplastic.
 21. Methodaccording to claim 20, wherein the thermoplastic of a cover part isselected from the group consisting of polyetherimide (PEI),polyethersulfone (PES), polyphenylsulfone (PPSU) and polysulfone (PSU).22. Method according to claim 12, wherein the thermoplastic of the corepart is polyetherimide (PEI).
 23. Method according to claim 21, whereinat least one of the first and second cover parts comprises a layer of afibre-reinforced thermoplastic.
 24. Method according to claim 14,wherein the second cooling rate is less than half of the first coolingrate.
 25. Method according to claim 14, wherein the second cooling rateis 25° C./min or less.
 26. Method according to claim 13, wherein duringsubsteps d1) and d2) the temperature difference between the press toolsis less than 2° C.
 27. Method according to claim 13, wherein the surfaceof the press tools is provided with a layer of a material having a heatconductivity coefficient, which is higher than that of the constructionmaterial from which the press tools are made.
 28. Method according toclaim 13, wherein the thermoplastic of the core part is polyetherimide(PEI).
 29. Method according to claim 14, wherein the thermoplastic ofthe core part is polyetherimide (PEI).
 30. The method of claim 28,wherein the second cooling rate is 25° C./min or less.
 31. The method ofclaim 29, wherein the second cooling rate is 25° C./min or less.